This invention relates to current transfer brushes in general and more particularly to a contact brush with several graphite fibers which are combined to form a slider member and are coated, at least partly, by an electrically conducting material.
The brushes used in electrical machines are used for current transfer between a fixed and a rotating machine part. Good electrical conductivity of the brush and, at the same time, good sliding characteristics on the contact member connected to the rotating machine part, such as a slip ring or a commutator, are assured through the use of graphite. The running characteristics of such a brush are determined mainly by the friction coefficient .mu. as a function of the circumferential velocity of the contact member connected to the rotating machine part and by the voltage drop .DELTA.U as function of the current density transferred via the brush. Both quantities depend to a large extent on the alien skin which forms on the rotating contact member and is also called film or patina. This alien skin is composed of materials of the brush slider member and of the contact member abraded during operation. Its thickness and nature are influenced by a multiplicity of factors. It is determined, for instance, by the material composition of the graphite and of the contact member, by the intended current density as well as by the circumferential velocity and the temperature of the contact member. It also depends on the contact pressure of the brush and, in particular, on the constantly changing influences of the atmosphere, such as ground and altitude climate, relative humidity, and chemically aggressive gases and vapors.
The slider members of such graphite brushes may contain a multiplicity of carbon or graphite fibers, combined to form a bundle and coated by a metal film of high electrical conductivity (British Pat. No. 1,191,234). Therein, graphite fibers in the form of a rope of several thousand individual fibers serve as starting material. Suitable fibers are known from British Pat. No. 1,110,791, for instance. The advantage of these fiber brushes over the known brushes having an electrographite block is that considerably more points of contact between the sliding member and the rotating contact surface are present, that the fibers are very elastic, and that the electrical characteristics and the running characteristics of the brush are thus improved.
The manufacture of such brushes and, above all, the metallizing of the graphite fibers is relatively costly, however. In addition, only a limited selection of metals can be applied to the graphite fibers by the known methods of currentless or galvanic deposition. Moreover, there are problems with these methods in achieving satisfactory adhesion and high conductivity. Precoating with a carrier material onto which the desired metal can only then be deposited may possibly be required.